Poly(lactic acid) has a high melting point and has highly been expected to be a biodegradable polymer melt-moldable and excellent in practical use. However, the poly(lactic acid) has a slow crystallization speed and is limited in crystallization and use for molded articles. For example, in the case of extrusion molding, not only long molding cycle time and heat treatment after molding is required but also the deformation is significant at the time of molding and heat treatment and thermostability is poor and thus there are serious problems for the practical use. Therefore, a method for making molding possible within a practically applicable molding cycle and improving the thermostability has been desired. Further, in the case of being used as a film, the poly(lactic acid) film has the highest tensile strength and modulus of elasticity and is regarded to be excellent in luster and transparency among various type of biodegradable films, however the glass transition temperature of the polymer is relatively low, so that it has been limited in the uses requiring thermostability. Accordingly, the thermostability (thermal deformation temperature) is required to be high.
On the other hand, cellulose and cellulose derivatives such as cellulosic esters and cellulose ether are biomass materials produced in the largest quantity all over the globe and are biodegradable polymers and therefore, they have drawn close attention. Further, as a method of melt molding these cellulose derivatives, a method described in Japanese Patent Application Laid-Open No. 53-11564 and comprising mixing a water soluble plasticizer such as polyethylene glycol with cellulose acetate and melt-spinning the resulting mixture has been known. However, use of a plasticizer with a high hygroscopic property such as polyethylene glycol is limited in the application and therefore it is undesirable and a widely applicable method is desired.
To mix two or more types of polymers has been known as a polymer blend or a polymer alloy and has been employed popularly as a method of improving the disadvantages of respective polymers. However, generally in the case of mixing two types of polymers, many polymers are separated in respective phases and it is common that one phase has an uneven coarse dispersion structure with several μm or larger. In such a dispersion state, many polymers become opaque and inferior in the mechanical strength and tend to cause Barus effect at the time of discharging in the case of melt kneading, resulting in poor productivity. On the other hand, although rarely, two type polymers are evenly mixed and these polymers are generally called compatible polymers or affinitive polymers and expected to have excellent properties, however such examples are limited.
Japanese Patent Application Laid-Open No. 11-241008 discloses methods of obtaining biodegradable polymer compositions having both flexibility and thermostability by adding natural polymers to resin compositions comprising polymer components of poly(lactic acid) and aliphatic polyesters and plasticizer. However in the document, as the natural polymer, starch (obtained from potato, corn, sweet potato, tapioca, and the like), chitin, chitosan, and celluloses are exemplified and as the celluloses, only acetyl cellulose, which is included in the cellulosic ester category, is exemplified and no practical example of adding the celluloses is given. In the cited document, there is no description given that poly(lactic acid) and acetyl cellulose are melting and kneaded to obtain excellent affinity or compatibility and excellent thermostability.
Further, Japanese Patent Application Laid-Open No. 2003-82160 and International application under PCT No. 92/09654 disclose methods of improving the moldability of cellulosic esters by blending aliphatic polyesters such as poly(lactic acid) with the cellulosic esters. However, in the documents, use of the aliphatic polyesters such as poly(lactic acid) as plasticizers for improving the moldability of cellulosic esters with high melt molding temperature is exemplified and there is no description of technical idea that poly(lactic acid) and acetyl cellulose are mixed to improve thermostability of the poly(lactic acid).
On the other hand, as methods of mixing poly(lactic acid) and polymers compatible with poly(lactic acid), Polymer, vol. 39(26), p.6891 (1998) and Micromol. Chem. Phys., vol. 201, p.1295 (2000) disclose addition of poly(methyl methacrylate) having a glass transition temperature about 100° C. for improving the glass transition temperature of the obtained polymer composition and also Japanese Patent Application Laid-Open No. 8-59949 discloses production of polymers excellent in hydrolyzability by mixing a-hydroxycarboxylic acid polymers including poly(lactic acid) and poly(methyl methacrylate) polymers and Japanese Patent Application Laid-Open No. 2002-155207 discloses-that acrylic compounds are added to poly(lactic acid) to obtain resin compositions excellent in weathering resistance and molding processibility and Japanese Patent Application Laid-Open No. 6-322217 discloses production of resin compositions excellent in water-proofness by mixing vinyl alcohol type polymers with polymers containing carbonyl compounds in the main chains, however any one discloses no technical idea relevant to improvement of the thermostability and high temperature rigidity and no implication of means for solution.
It could, therefore, be helpful to provide a poly(lactic acid) polymer composition excellent in the transparency, mechanical properties, and thermostability, a biaxially drawn film, and a molded article.